Linear accelerators have been in use for a significant time for the treatment of severe medical conditions such as cancer. The principle of operation of these devices is essentially that electromagnetic radiation of sufficiently high energy (greater than 1 MeV) causes the death and/or destruction of living cells in its path. If therefore a beam of high energy photons or electrons is directed to a cancerous area, the cancer will be destroyed.
It is clear that this treatment has the potential to cause harm to the patient if the radiation is incident on healthy tissue. It is therefore common practice to collimate the beam in order to limit its spatial extent and therefore limit the amount of tissue, healthy or otherwise, upon which it is incident. Collimators to this effect are known, and include collimators of variable size and shape which are capable of selectively blocking the radiation beam at its edges thereby to produce a beam whose shape corresponds to that of the tumour.
In this situation, it is clearly necessary to ensure that the positioning of the patient is accurate. This is generally done by shining one or more collimated light sources on the patient, such as low energy lasers. When the patient is first treated, ink marks are placed on the patient's skin and these are subsequently used to align the patient relative to the lasers. This is able to provide a positioning accuracy of approximately 2-5 mm.
As a result, the shaped radiation beam needs to be formed to the approximate outline shape of the tumour plus a margin in each direction to allow for positioning error. This results in a significant volume of tissue around the tumour which is needlessly irradiated. If the patient could be positioned more accurately beneath the beam, then this margin around the tumour could be reduced thereby limiting the damage caused to the patient's healthy tissue.
It is known to prepare a "Portal image" derived from the radiation transmitted through the patient and incident on a photographic plate or other detector. However, the absorption co-efficients of bone, tissue etc. at the energies normally used for radiotherapy are very similar and therefore a Portal image shows very little detail.